Rieju wiring diagram - 2011 Rieju Marathon Pro 125

Can't help with a wiring diagram. Lots of bikes built in the last couple of decades are fitted with an immobiliser and they are far from trouble-free. The first step when the engine doesn't start is to check the immobiliser is disarmed - if the bike is equipped with one. Often that information is found only in the owner's handbook or at the dealer.

Although wiring harnesses aren't very long on bikes there is usually quite a lot of it and manufacturers don't make it any better than it absolutely has to be and there are places where wiring can be stretched or chafed and where it jumps from the frame to the steering there is lots of flexing.
I suggest you expose as much of the wiring as you can and inspect it carefully for signs of damage. Check whether the fuse(s) are ok and have power on them will give you a start.

https://www.riejuparts.co.uk/

If you perform a search for the rieju marathos pro 125sm and click on IMAGES rather than ALL, you will enounter a number of diagrams of possible interest to you. But you may have to search ebay for such a manual. You do have to be careful to sort through them for this particular Rieju Marathon. Whether or not the parts link listed above might also have wiring diagrams available would require contact with them. You might have to inquire with a dealer for such if indeed them have such access. Here are links of possible help.

https://www.riejumotorcycles.co.uk/m/9/marathon-125-sm-ac

http://www.riejumoto.co.uk/documents.php?type=5

https://www.oemmotorparts.com/oem2.asp?M=rieju

https://www.manualslib.com/manual/958870/Rieju-Marathon-125.html

https://bikez.com/motorcycles/rieju_marathon_pro_125_2011.php

https://en.wikipedia.org/wiki/Rieju

Put some BG or amsoil PI in tank..littl ata time

Maybe you can find your manual on carlselter.com. I found a lot on this Web site. Good luck.

Well the horn could be very well grounding out and causing that problem or there is a faulty wire or short running with the horn check all connections for corrosion as well and clean components and re connect everything to see if it stops your problem